Fluid flow meters

ABSTRACT

A flow meter for measuring the flow rate of an electrically conductive fluid, particularly blood, which comprises a tube, an electromagnet consisting of a ferro-magnetic core material in the form of a thin-walled collar encircling or substantially encircling the tube and windings on the core, the windings being wound transversely around the core in a uniform manner but being oppositely directed on diametrically opposed halves of the core, and a plurality of electrodes adapted to be in electrical contact with the following fluid and so arranged that in operation an induced potential will exist between the electrodes located in each half of the electromagnet when the fluid flows through the tube and a current is passed through the windings.

United. States Patent Wyatt [54] FLUID FLOW METERS [72] Inventor: DerekGerald Wyatt, Oxford, En-

gland Assignee: National Research Development Corporation, London,England Filed: April 30, 1971 Appl. No.: 139,272

Related US. Application Data Continuation of Ser. No. 822,204, May 6,1969.

US. Cl ..73/l94 EM, 128/205 F Int. Cl. ..G0lp 5/08 Field of Search ..73/194 EM; l28/2.05 F

References Cited UNITED STATES PATENTS 3,605,726 9/1971 Williams etal...73/194EM 3,681,986 [451 Aug. 8, 1972 FOREIGN PATENTS OR APPLICATIONS1,095,915 12/1967 Great Britain ..73/l94 EM Primary Examiner-Charles A.Ruehl Attomey-Cushman, Darby and Cushman ABSTRACT A flow meter formeasuring the flow rate of an electriadapted to be in electrical contactwith the following fluid and so arranged that in operation an inducedpotential will exist between the electrodes located in each half of theelectromagnet when the fluid flows through the tube and a current ispassed through the. windings.

6Claims,2DrawingFigures FLUID FLOW METERS This is a continuation ofapplication, Ser. No. 822,204, filed May 6, 1969.

This invention relates to fluid flow meters and more particularly to aflow meter of the type in which fluid flow is measured indirectly byallowing the fluid to flow in a magnetic field using e.m.f. induced inthe fluid as a measure of fluid flow.

In existing fluid flow meters of this type, the magnetic field has beengenerated by an electromagnet in which the windings are on aconventional iron U-core or which has no core or in which the windingsare concentrated in one region of an iron core and which are notdistributed uniformly over it. Such meters are not equally sensitive tolaminar and turbulent fluid flow. It has now been found that thisproblem can be alleviated particularly in tubular blood flow meters, bymodifying the magnetic field pattern.

' The present invention provides a flow meter for measuring the flowrate of an electrically conductive fluid, particularly blood, whichcomprises a tube, an electromagnet consisting of a ferro-magnetic corematerial in the form of a thin-walled collar encircling or substantiallyencircling the tube and windings on the core, the windings being woundtransversely around the core in a uniform manner but being oppositelydirected on diametrically opposed halves of the core, and a plurality ofelectrodes adapted to be in electrical contact with the flowing fluidand so arranged that in operation an induced potential will existbetween the electrodes The size of the strips influences the degree ofreduction of magnetic intensity and some experimentation is necessary todeterminethe optimum size for a particular application. Magneticintensity may also be reduced by creating a gap in the windings in theregion of the electrical center of the electrode systems or providing atsuch points suitable ancillary windings.

The electrodes may be of a noble metal, particularly platinum, having abright surface or a surface of deposited colloidal noble metal. Theseelectrodes may be located in electrical connection with the wall of thevessel through which the fluid flows and are preferably recessed, insidethis wall in suitable cases.

A suitable embodiment of this invention will now be described, withreference to the accompanying draw ing, in which:

located in each half of the electromagnet when the fluid flows throughthe tube and a current is passed through the windings.

The flow meter is preferably of cylindrical form with an axiallydirected slot communicating the interior and the exterior of the meterso that an intact artery or vein can be inserted directly.Alternatively, the cylinder may be intact and provided with endssuitable for the attachment of divided arteries, veins or other tubing.

The electromagnet core has preferably a high magnetic permeability andlaminated Permalloy C is particularly suitable.

The direction of the windings around the metal core is reversed half wayround, conveniently at the point diametrically opposite to the openingof slotted meters, and when the windings are excited by an AC. current asubstantially North/South magnetic field is obtained.

It has been found that the regions inside the meter most affected byflow rate and by magnetic field strength are those regions at the endsof a diameter drawn perpendicular to the general direction of themagnetic field and the electrodes are preferably located in this region.Two electrodes may be used, one at each end of the above-mentioneddiameter, but more than two can be used in which case the center of eachelectrode system should be located at or near the ends of theabove-mentioned diameters.

The magnetic intensity is higher in the region of the electrodesrelative to that at the axis of the meter and benefit can be obtained tothe uniformity of sensitivity of the meter with changing flow profile byreducing the magnetic intensity in the region of the electrodes. Thisreduction is preferably achieved by providing strips of a ferro-magneticmaterial, such as Permalloy B, located between the electrode and themagnetic system.

FIG. 1 shows a perspective view of a slotted cylindrical blood meter;and

FIG. 2 shows an end elevation.

The instrument comprises a hollow tube or cannula T through which bloodispermitted to flow or within which an intact artery or vein may beplaced by passing the vessel through slot AA. The cannula is fitted withelectrodes E E which are of platinised gold, which are, for better baseline stability, placed on the bottom of shallow slots circumferentiallydirected and subtending an angle of about 20 at the axis of the cannula.The latter arrangement has the further advantage of low noise level. Themagnetic field is provided by an energized magnetic circuit, consistingof a laminated core of Permalloy C (CC') around which is wound a numberof turns of wire W which carry the exciting current. The turns are woundin opposing directions on each half of the core, thus producing amagnetic flux, the general direction of which is North/South. Thedistribution of magnetic flux within the cannula is such that themagnetic intensity at the electrodes E E is greater than that at theaxis whereas the magnetic intensity in the region of the slot AA is lessthan that at the axis, that at the closed side P is greater than that atthe axis. The net effect of this magnetic field distribution is to causeincreased sensitivity to flow when the latter is turbulent as comparedwith that when the flow is lamina. This difference may be reduced to afraction of 1 percent by reducing the magnetic intensity at the of 3.3mm. gives nearly the same sensitivity for both laminar and turbulentflows and for intermediate regimes while axis symmetry of flow profileis maintained.

The device described above is designed as a blood flow meter but theprinciple of operation is equally applicable to the measurement of flowrate of other electrically conducting fluids.

The magnetic circuit described above has the following advantages:

1. Volume within its cylindrical walls is largely filled with iron andcopper and, therefore, is largely usefully employed from anelectromagnetic viewpoint. Further- 3 more, the area of thewindingscoincides with the inner and outer areas of the cylinder,leading to efiicient heat dissipation and minimal temperature rise ofthe windings. As a result, the flow meter head can be made with asmaller wall thickness, lower temperature rise and higher sensitivitythan the conventional Aircored head. They also compare favorably withthe conventional U-shaped iron core heads partly because the leakageflux, and therefore the necessary cross-sec tion, is less.

2. The heads can be designed over a large range of sizes from 3 mm.lumen diameter upwards and the design formulas can be programmed and thecharac-' teristics computed in detail.

3. The magnetic circuit is readily screened in itsentirety, thuseliminating spurious signals, this can be done by wrappingcircumferentially within and without the magnetic circuit, a strip ofhigh resistivity alloy-of small thickness, e.g. 0.0004 inch 45 percentnickel copper.

4. The electrode leads are led, when they are not twisted together,within themagnetic circuit which produces a useful degree of screeningfrom external magnetic fields, e.g. the leakage flux from adjacentelectromagnetic flow meter heads, and interaction between heads isreduced. Y 1

I claim:

1. In a flowmeter for measuring the flow rate of blood through a tube,said meter comprising a core of ferro-magnetic material in the form of athin-walled 4 induced potential will exist between the electrodes whenfluid flows through said tube and a current is passed through windings,the improvement com- Pflsmgi shielding strips of ferro-magnetic materialdisposed substantially adjacent and between said electrodes and themagnetic system comprising said core and said windings for modifying themagnetic intensity in the region of the electrodes by concentrating theavailable magnetic intensity in said strips thereby making the magneticfield distribution in said tube even less uniform by substantiallyreducing the magnetic field intensity in the immediate vicinity of saidelectrodes within said fluid and thereby reducing the susceptibility ofsaid flowmeter to changes in sensitivity caused by a changingaxisymmetric flow profile of the fluid.

2. An improved blood flowmeter as in claim 1 wherein the electrodes arelocated inside the magnetic system in regions at the ends of a diameterdrawn per pendicular to the direction of the magnetic field.

-3. An improved blood flowmeter as in claim 1 wherein the flowmeter isof the cufl type having a generally cylindrical form with an axiallydirected slot collar encircling or substantially encircling said tube,electrical windings wound transversely about said corein a uniformmanner 'but being oppositely directed on diametrically opposed halves ofthe core to produce a nonuniform magnetic field said tube, and aplurality of electrodes adapted for electrical contact with the flowingfluidand so arranged that, in operation, an

communicating between; the interior and exterior of the meter forinsertion of a blood vessel.

4. An improved blood flowmeter as in claim 3 wherein the directio of eare reversed at a paint openingof the slot.

opposlte to e 5. An improved blood flowmeter asin claim lv wherein saidferro-magnetic material comprises laminated Permalloy C. Y

6. An improved blood flowmeter as in claim I wherein the flowmeter is-ofthe cannular type having a central tube for fluid connection with adivided blood vessel.

about the co e

1. In a flowmeter for measuring the flow rate of blood through a tube,said meter comprising a core of ferro-magnetic material in the form of athin-walled collar encircling or substantially encircling said tube,electrical windings wound transversely about said core in a uniformmanner but being oppositely directed on diametrically opposed halves ofthe core to produce a nonuniform magnetic field within said tube, and aplurality of electrodes adapted for electrical contact with the flowingfluid and so arranged that, in operation, an induced potential willexist between the electrodes when fluid flows through said tube and acurrent is passed through said windings, the improvement comprising:shielding strips of ferro-magnetic material disposed substantiallyadjacent and between said electrodes and the magnetic system comprisingsaid core and said windings for modifying the magnetic intensity in theregion of the electrodes by concentrating the available magneticintensity in said strips thereby making the magnetic field distributionin said tube even less uniform by substantially reducing the magneticfield intensity in the immediate vicinity of said electrodes within saidfluid and thereby reducing the susceptibility of said flowmeter tochanges in sensitivity caused by a changing axisymmetric flow profile ofthe fluid.
 2. An improved blood flowmeter as in claim 1 wherein theelectrodes are located inside the magnetic system in regions at the endsof a diameter drawn perpendicular to the direction of the magneticfield.
 3. An improved blood flowmeter as in claim 1 wherein theflowmeter is of the cuff type having a generally cylindrical form withan axially directed slot communicating between the interior and exteriorof the meter for insertion of a blood vessel.
 4. An improved bloodflowmeter as in claim 3 wherein the direction of the windings about thecore are reversed at a point diametrically opposite to the opening ofthe slot.
 5. An improved blood flowmeter as in claim 1 wherein saidferro-magnetic material comprises laminated Permalloy C.
 6. An improvedblood flowmeter as in claim 1 wherein the flowmeter is of the cannulartype having a central tube for fluid connection with a divided bloodvessel.